Cu/Zn superoxide dismutase (SOD!) is a well-conserved anti-oxidant enzyme present in virtually all eukaryotic cells and tissues. While clearly important for oxidative stress, little is known about the biology of SODl under physiologically relevant oxygen conditions. The overall goal of this research is to understand how SODl is controlled by changes in cellular redox and how SODl and its activator protein CCS maintain cell fitness over a range of oxygen tensions. We identified CCS as the key activator of human SODl under atmospheric oxygen, but when oxygen levels drop, SODl is activated by a CCS independent pathway. We hypothesize this switch in SODl activation has evolved as adaptation to varying redox environments of tissues and sub-cellular compartments, and we will address this using both yeast and mammalian expression systems. We identified the SODl disulfide as the target for redox control of SOD], but how the disulfide is oxidized under hypoxic conditions without CCS is not clear. Yeast genetic screens will be used to identity the trans-acting factors that work independent of CCS to target the disulfide and activate SOD]. Cu/Zn SODl is not normally essential for aerobic life, but we observed that yeast sodl null mutants cannot thrive in air when levels of cytosolic phosphate rise. Phosphate-metal interactions may be key to this severe oxidative stress and through molecular/genetic approaches, we will explore the mechanism of phosphate mediated oxygen toxicity and how this damage is ameliorated by SODl. Finally we observed an interesting effect of hypoxia on SODl and CCS. SODl activity and protein levels drop dramatically during hypoxia in yeast, while CCS levels remain high. We found that this hypoxic loss of SODl can be important for anaerobic cell survival. Hypoxia can trigger mitochondrial production of reactive oxygen and nitrogen species for signaling, and we will test the hypothesis that loss of mitochondrial SODl is a critical component of this hypoxic signaling. The rationale for maintaining CCS with low oxygen is not clear, but we observe an effect of CCS loss on genomic stability even under anaerobic conditions. We will continue to explore this novel effect of CCS on chromatin using molecular-genetic strategies. All together, these studies promise to shed new important light into the biology of an important anti-oxidant enzyme and its activator protein CCS.